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991.
Christopher S. Lowry Jeffrey S. Deems Steven P. Loheide II Jessica D. Lundquist 《水文研究》2010,24(20):2821-2833
Quantifying snowmelt‐derived fluxes at the watershed scale within hillslope environments is critical for investigating local meadow scale groundwater dynamics in high elevation riparian ecosystems. In this article, we investigate the impact of snowmelt‐derived groundwater flux from the surrounding hillslopes on water table dynamics in Tuolumne Meadows, which is located in the Sierra Nevada Mountains of California, USA. Results show water levels within the meadow are controlled by a combination of fluxes at the hillslope boundaries, snowmelt within the meadow and changes in the stream stage. Observed water level fluctuations at the boundaries of the meadow show the hydrologic connection and subsequent disconnection between the hillslope and meadow aquifers. Timing of groundwater flux entering the meadow as a result of spring snowmelt can vary over 20 days based on the location, aspect, and local geology of the contributing area within the larger watershed. Identifying this temporal and spatial variability in flux entering the meadow is critical for simulating changes in water levels within the meadow. Model results can vary significantly based on the temporal and spatial scales at which watershed processes are linked to local processes within the meadow causing errors when boundary fluxes are lumped in time or space. Without a clear understanding of the surrounding hillslope hydrology, it is difficult to simulate groundwater dynamics within high elevation riparian ecosystems with the accuracy necessary for understanding ecosystem response. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
992.
Leah Meromy Noah P. Molotch Timothy E. Link Steven R. Fassnacht Robert Rice 《水文研究》2013,27(17):2383-2400
The spatial distribution of snow water equivalent (SWE) is a key variable in many regional‐scale land surface models. Currently, the assimilation of point‐scale snow sensor data into these models is commonly performed without consideration of the spatial representativeness of the point data with respect to the model grid‐scale SWE. To improve the understanding of the relationship between point‐scale snow measurements and surrounding areas, we characterized the spatial distribution of snow depth and SWE within 1‐, 4‐ and 16‐km2 grids surrounding 15 snow stations (snowpack telemetry and California snow sensors) in California, Colorado, Wyoming, Idaho and Oregon during the 2008 and 2009 snow seasons. More than 30 000 field observations of snowpack properties were used with binary regression tree models to relate SWE at the sensor site to the surrounding area SWE to evaluate the sensor representativeness of larger‐scale conditions. Unlike previous research, we did not find consistent high biases in snow sensor depth values as biases over all sites ranged from 74% overestimates to 77% underestimates. Of the 53 assessments, 27 surveys indicated snow station biases of less than 10% of the surrounding mean observed snow depth. Depth biases were largely dictated by the physiographic relationship between the snow sensor locations and the mean characteristics of the surrounding grid, in particular, elevation, solar radiation index and vegetation density. These scaling relationships may improve snow sensor data assimilation; an example application is illustrated for the National Operational Hydrologic Remote Sensing Center National Snow Analysis SWE product. The snow sensor bias information indicated that the assimilation of point data into the National Operational Hydrologic Remote Sensing Center model was often unnecessary and reduced model accuracy. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
993.
Steven Suranovic 《Global Environmental Change》2013,23(3):598-608
This paper applies a behavioral economics model of cigarette addiction to the issue of fossil fuel usage and climate change. Both problems involve consumption of a currently beneficial product that causes detrimental effects in the distant future and for which current reductions in usage induces an adjustment cost. The paper argues that because fossil fuel control requires solving an international public goods problem as well as an addiction-like problem, breaking it will be more challenging. Using insights from the model, it also suggests that fossil fuel addiction, like cigarette addiction, may generate a long period of time in which people express sincere desire to convert to clean energy, but accomplish little to achieve that outcome. Finally the paper examines the history of the international anti-smoking campaign to draw insights for the campaign against global climate change. The analogy suggests that policies to reduce the present cost of non-carbon energy sources to induce voluntary adjustments in energy usage, or, policies that induce cleaner usage of fossil fuels, or geo-engineering policies that work to reverse the warming effects of higher CO2 concentrations, may be more viable than policies that raise the cost of current fossil fuel consumption. 相似文献
994.
Kieran M. Baker Simon H. Bottrell Steven F. Thornton Kate E. Peel Michael J. Spence 《Applied Geochemistry》2012
The availability of dissolved O2 can limit biodegradation of organic compounds in aquifers. Where O2 is depleted, biodegradation proceeds via anaerobic processes, including NO3-, Mn(IV)-, Fe(III)- and SO4-reduction and fermentation/methanogenesis. The environmental controls on these anaerobic processes must be understood to support implementation of management strategies such as monitored natural attenuation (MNA). In this study stable isotope analysis is used to show that the relative significance of two key anaerobic biodegradation processes (bacterial SO4 reduction (BSR) and methanogenesis) in a phenol-contaminated sandstone aquifer is sensitive to spatial and temporal changes in total dissolved phenols concentration (TPC) (= phenol + cresols + dimethylphenols) over a 5-a period. In general, 34SO4-enrichment (characteristic of bacterial SO4 reduction) is restricted spatially to locations where TPC < 2000 mg L−1. In contrast, 13C-depleted CH4 and 13C-enriched CO2 isotope compositions (characteristic of methanogenesis) were measured at TPC up to 8000 mg L−1. This is consistent with previous studies that demonstrate suppression of BSR at TPC of >500 mg L−1, and suggests that methanogenic microorganisms may have a higher tolerance for TPC in this contaminant plume. It is concluded that isotopic enrichment trends can be used to identify conditions under which in situ biodegradation may be limited by the properties of the biodegradation substrate (in this case TPC). Such data may be used to deduce the performance of MNA for contaminated groundwater in similar settings. 相似文献
995.
Steven Franke Hannes Eisermann Wilfried Jokat Graeme Eagles Jölund Asseng Heinrich Miller Daniel Steinhage Veit Helm Olaf Eisen Daniela Jansen 《地球表面变化过程与地形》2021,46(13):2728-2745
The landscape of Antarctica, hidden beneath kilometre-thick ice in most places, has been shaped by the interactions between tectonic and erosional processes. The flow dynamics of the thick ice cover deepened pre-formed topographic depressions by glacial erosion, but also preserved the subglacial landscapes in regions with moderate to slow ice flow. Mapping the spatial variability of these structures provides the basis for reconstruction of the evolution of subglacial morphology. This study focuses on the Jutulstraumen Glacier drainage system in Dronning Maud Land, East Antarctica. The Jutulstraumen Glacier reaches the ocean via the Jutulstraumen Graben, which is the only significant passage for draining the East Antarctic Ice Sheet through the western part of the Dronning Maud Land mountain chain. We acquired new bed topography data during an airborne radar campaign in the region upstream of the Jutulstraumen Graben to characterise the source area of the glacier. The new data show a deep relief to be generally under-represented in available bed topography compilations. Our analysis of the bed topography, valley characteristics and bed roughness leads to the conclusion that much more of the alpine landscape that would have formed prior to the Antarctic Ice Sheet is preserved than previously anticipated. We identify an active and deeply eroded U-shaped valley network next to largely preserved passive fluvial and glacial modified landscapes. Based on the landscape classification, we reconstruct the temporal sequence by which ice flow modified the topography since the beginning of the glaciation of Antarctica. 相似文献
996.
A framework for understanding the hydroecology of impacted wet meadows in the Sierra Nevada and Cascade Ranges, California, USA 总被引:1,自引:0,他引:1
Steven P. Loheide II Richard S. Deitchman David J. Cooper Evan C. Wolf Christopher T. Hammersmark Jessica D. Lundquist 《Hydrogeology Journal》2009,17(1):229-246
Meadows of the Sierra Nevada and Cascade mountains of California, USA, support diverse and highly productive wet-meadow vegetation dominated by sedges, rushes, grasses, and other herbaceous species. These groundwater–dependent ecosystems rely on the persistence of a shallow water table throughout the dry summer. Case studies of Bear Creek, Last Chance, and Tuolumne meadow ecosystems are used to create a conceptual framework describing groundwater–ecosystem connections in this environment. The water requirements for wet-meadow vegetation at each site are represented as a water-table-depth hydrograph; however, these hydrographs were found to vary among sites. Causes of this variation include (1) differences in soil texture, which govern capillary effects and availability of vadose water and (2) elevation-controlled differences in climate that affect the phenology of the vegetation. The field observations show that spatial variation of water-table depth exerts strong control on vegetation composition and spatial patterning. Groundwater-flow modeling demonstrates that lower hydraulic-conductivity meadow sediments, higher groundwater-inflow rates, and a higher ratio of lateral to basal-groundwater inflow all encourage the persistence of a high water table and wet-meadow vegetation, particularly at the margin of the meadow, even in cases with moderate stream incision. 相似文献
997.
Terrestrial vegetation dynamics and global climate controls 总被引:2,自引:0,他引:2
Christopher Potter Shyam Boriah Michael Steinbach Vipin Kumar Steven Klooster 《Climate Dynamics》2008,31(1):67-78
Monthly data from the moderate resolution imaging spectroradiometer (MODIS) and its predecessor satellite sensors was used to reconstruct vegetation dynamics in response to climate patterns over the period 1983–2005. Results suggest that plant growth over extensive land areas of southern Africa and Central Asia were the most closely coupled of any major land area to El Niño–southern oscillation (ENSO) effects on regional climate. Others land areas strongly tied to recent ENSO climate effects were in northern Canada, Alaska, western US, northern Mexico, northern Argentina, and Australia. Localized variations in precipitation were the most common controllers of monthly values for the fraction absorbed of photosynthetically active radiation (FPAR) over these regions. In addition to the areas cited above, seasonal FPAR values from MODIS were closely coupled to rainfall patterns in grassland and cropland areas of the northern and central US. Historical associations between global vegetation FPAR and atmospheric carbon dioxide (CO2) anomalies suggest that the terrestrial biosphere can contribute major fluxes of CO2 during major drought events, such as those triggered by 1997–1998 El Niño event. 相似文献
998.
Bryan Shuman Anna K. Henderson Steven M. Colman Jeffery R. Stone Sherilyn C. Fritz Lora R. Stevens Mitchell J. Power Cathy Whitlock 《Quaternary Science Reviews》2009,28(19-20):1861-1879
The availability of water shapes life in the western United States, and much of the water in the region originates in the Rocky Mountains. Few studies, however, have explicitly examined the history of water levels in the Rocky Mountains during the Holocene. Here, we examine the past levels of three lakes near the Continental Divide in Montana and Colorado to reconstruct Holocene moisture trends. Using transects of sediment cores and sub-surface geophysical profiles from each lake, we find that mid-Holocene shorelines in the small lakes (4–110 ha) were as much as 10 m below the modern lake surfaces. Our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000–2000 years ago. We also discuss evidence for millennial-scale moisture variation, including an abruptly-initiated and -terminated wet period in Colorado from 4400 to 3700 cal yr BP, and find only limited evidence for low-lake stands during the past millennium. The extent of low-water levels during the mid-Holocene, which were most severe and widespread ca 7000–4500 cal yr BP, is consistent with the extent of insolation-induced aridity in previously published regional climate model simulations. Like the simulations, the lake data provide no evidence for enhanced zonal flow during the mid-Holocene, which has been invoked to explain enhanced mid-continent aridity at the time. The data, including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium, confirm the ability of large boundary-condition changes to push western water supplies beyond the range of recent natural variability. 相似文献
999.
Steven Goderis Elin Kalleson Roald Tagle Henning Dypvik Ralf-Thomas Schmitt Jörg Erzinger Philippe Claeys 《Chemical Geology》2009,258(3-4):145-156
The goal of this study is to identify the type of projectile responsible for the formation of the late Precambrian Gardnos impact structure in Norway. Fifteen impactite samples, predominantly impact breccias and suevites from the central and northeastern part of the structure, were analyzed for platinum group elements (PGE) and Au using nickel-sulfide fire assay combined with inductively coupled plasma mass spectrometry (ICP-MS). Major and trace elements were measured in the same samples using X-ray fluorescence (XRF). In addition, the concentrations of siderophile elements Ni, Cr, and Co were determined by ICP-MS after acid digestion. The samples collected at the contact between suevite and the sedimentary infill yielded the highest PGE concentrations (Ir = 1.926 ng/g, Ru = 3.494 ng/g, Pt = 4.716 ng/g, Rh = 0.766 ng/g, Pd = 2.842 ng/g for GC6). The CI-normalized PGE patterns are characterized by Ru and Rh enrichments suggesting a non-chondritic impactor. Concentration plots of the different PGE display an excellent correlation (R > 0.99), indicative of a single source for the PGE enrichment. The Ni/Cr ratio of the Gardnos impactor (2.56 ± 0.20) agrees with that of chondrites (2 to 7), whereas Ir is depleted relative to Ni in this projectile (Ni/Ir ratio of 92 000 ± 8000 compared to an average Ni/Ir ratio of 23 150 ± 4250 for chondrites). There is no clear indication of selective post-depositional remobilization of the characteristic highly siderophile elements. The Ni/Ir and Cr/Ir data combined with the non-chondritic PGE ratios probably indicate a differentiated projectile. Based on (1) the similarity of the inter-element ratios of the impactor with the iron phase of non-magmatic iron meteorites and (2) the presence of characteristics of both chondrites and iron meteorites (Ni/Cr and Ni/Ir ratios), an IA or IIIC non-magmatic iron meteorite is a very plausible impactor. 相似文献
1000.
Brian R. Jicha Garret L. Hart Clark M. Johnson Wes Hildreth Brian L. Beard Steven B. Shirey John W. Valley 《Contributions to Mineralogy and Petrology》2009,157(2):189-207
Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and
its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources,
a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns
for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade
arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators
of slab-fluids such as (Sr/P)
n
and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle
source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification
is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most
Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and
isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass
of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization
in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher
than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ
18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding
involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope
compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile
arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust. 相似文献